1. Field of the Invention
This invention relates to domestic water heaters used with circulating boilers, particularly in a combination water and space heating system.
2. Discussion of the Background
It is possible to utilize a circulating boiler in a system which provides both space heat and domestic hot water. Such systems are often used in large commercial, industrial, and institutional buildings, and less frequently in residential and light commercial buildings. The present invention is intended primarily for the latter application.
A combination system based on a circulating boiler can be configured in several ways. These different approaches fall into two broad categories: open loop systems and closed loop systems. In an open loop system, potable water is utilized as the circulating boiler fluid, and hot water taps are branched directly from the boiler loop. It is characteristic of such a system that the boiler circulates a constantly changing supply of water as hot water draws are made from the loop and cold supply water replaces it. It is also characteristic of such a system that the water circulated for the purpose of space heating must have the same temperature as that of the potable hot water.
In the closed loop system, the boiler loop is separated from the domestic hot water system, and an unchanging supply of fluid is circulated in the boiler loop. In a closed loop combination system, domestic hot water is generated by a heat exchanger whose function it is to maintain physical separation between the circulating boiler fluid and the domestic water supply. A closed loop system is somewhat more complex than an open loop system, but it offers three advantages:
1. Mineral buildup in the boiler loop is eliminated. PA0 2. The boiler loop can operate at a higher temperature. PA0 3. Fluid other than water, such as steam, brine, or antifreeze solution, can be used in the boiler loop. PA0 1. Storage tank water heaters PA0 2. Instantaneous water heaters PA0 3. Semi-instantaneous water heaters.
The advantage of operating the boiler loop at a higher temperature (say 200 F.) is that if radiators or convectors are used for space heat, less heat transfer area is required to move a given amount of heat than if the boiler loop is limited to normal domestic hot water temperature (about 140 F.).
There have been several approaches to heat exchanger design for generating domestic hot water in closed loop combination systems. These approaches can be broadly categorized as follows:
In the first approach, a heat exchanger is immersed in a relatively large tank. This heat exchanger is usually a tube coil; the tube may be either finned or unfinned. A further characteristic of such a system is that the tank-side fluid is relatively quiescent as far as the heat transfer regime is concerned. In the storage tank heater, no effort is made to promote fluid velocity over the heat exchange surface on the tank side; therefore free convection is the predominant tankside heat transfer mechanism. The storage tank heater is therefore characterized by a modest rate of heat transfer relative to the volume of water stored, and hot water demand is met largely by stored capacitance. The best way to plumb such a system is to circulate boiler fluid in the tube coil and store domestic hot water in the tank. One advantage of the storage tank water heater is inherent temperature stability in the hot water supply due to the large thermal capacitance of the stored hot water. Another advantage is that a single-input (nonmodulating) boiler may be used. A third advantage is that a large flowrate may be tapped, at least until the tank is drained of hot water and the boiler cannot keep up with the demand. The disadvantage is that a large tank must be used, with the associated cost, bulk, and thermal loss. Sometimes, the boiler fluid is circulated through the tank and the domestic water is plumbed through the immersed tube coil. Unfortunately, this arrangement retains the disadvantages of the storage tank while reaping little of the benefit. The thermal capacitance is not put to good use, since at high hot water draw, heat will not be transferred at a rate sufficient to maintain hot water temperature unless the coil area is made very large.
The instantaneous water heater is a heat exchanger without any appreciable volume, in which heat is transferred from the boiler fluid flowing through on one side to the domestic water flowing through on the other side. Typically, high fluid velocity is maintained on both sides of the heat exchanger, augmenting the heat transfer coefficient and making possible a compact design relative to the heat transfer rate capacity of the unit. Typical of these compact heat exchangers are tube-in-tube and shell-and-tube designs. Operationally, the system must have a means to sense hot water draw (a flow switch). The boiler circulation pump and ignition system are energized when water flow is sensed. Also, an automatically modulating boiler is mandatory in this system, since there is little thermal capacitance. The heat input to the boiler must closely follow the heating rate required for the hot water draw rate. Temperature instability due to rapid changes in hot water flowrate is inevitable in this system, and the best that can be hoped for in the design of the control system is to keep such instability to a reasonable level. The advantage of the instantaneous water heater is that no hot water is stored, so that there is no corresponding thermal loss. The disadvantages are system complexity and control difficulty. Another disadvantage is that the boiler is ill-suited to respond to demand spikes, in which a hot water tap is opened for a short period and then closed. With the instantaneous water heater, a series of demand spikes causes the boiler to ignite and shut down in rapidfire sequence, which is an undesirable operational mode. A boiler operates best at steady-state or quasi steady-state; during startup and shutdown, gas is wasted. Therefore, it is advantageous to avoid excessive boiler on/off cycling.
The semi-instantaneous water heater is an approach that is in between the preceding two. It realizes in some measure the advantages of each while minimizing the disadvantages. In this approach, a compact forced convection heat exchanger is used with a small storage tank of hot water which provides some thermal capacitance. The tank-heat exchanger system is designed so that heat can also be transferred from circulating boiler fluid to quiescent water in the tank when there is no domestic water flow through the heat exchanger. Therefore, the heat exchanger can operate in two modes: in the flow (forced convection) mode, heat is transferred at a high rate, thereby providing the capability for delivering an endless flow of hot water; in the recharge (free convection) mode, heat is transferred at a lower rate to quiescent water in the tank, thereby maintaining a small volume of stored hot water. There are several advantages related to maintaining this stored volume of hot water. A modulating boiler must be used in this system as it is with the instantaneous water heater, but the thermal capacitance dampens out the temperature instabilities associated with the instantaneous water heater. It also permits a looser link between the boiler heating rate and the heating rate associated with the rate of hot water draw, thereby making controller design easier. In fact, with the semi-instantaneous water heater, the flow switch can be eliminated, and hot water temperature in the heater tank can be used as the feedback control variable. The thermal capacitance also eases considerably the boiler cycling problem that can arise from demand spikes.
The present invention is a semi-instantaneous water heater of novel design. When used with a modulating boiler, particularly in a combination space and water heating system, it provides the advantages summarized above. These and other advantages of the invention will be discussed in the following section.